1. Field of the Invention
The present invention relates generally to an inorganic, dielectric, absorptive grid polarizer with particular focus on such a polarizer for use in the ultra-violet (UV) portion of the electromagnetic spectrum.
2. Related Art
Various types of polarizers or polarizing beam splitters (PBS) have been developed for polarizing light, or separating orthogonal polarization orientations of light. A MacNeille PBS is based upon achieving Brewster's angle behavior at the thin film interface along the diagonal of the high refractive index cube in which it is constructed. Such MacNeille PBSs generate no astigmatism, but have a narrow acceptance angle, and have significant cost and weight. Such devices can be fabricated to function from the infra-red through the visible to the ultra-violet region of the electromagnetic spectrum by appropriate choices of glasses and thin-films.
Other types of polarizers are also available for the visible and infra-red portions of the spectrum, including long-chain polymer polarizers, wire-grid polarizers, Glan Thompson crystal polarizers, etc. However, the ultra-violet (UV) portion of the spectrum, especially for wavelengths less than approximately 350 nm, is not similarly well-supplied with capable, high-performance polarizers.
This scarcity of capable polarizers has limited the applications of polarized UV light in science, technology, and industry in comparison to the visible and infra-red (IR). The need for UV polarizers, however, is becoming acute in order to support the increasing applications of UV irradiation in industrial processes such as semiconductor manufacturing, flat panel Liquid Crystal Display (LCD) manufacturing, etc. The type of polarizer needed in some UV irradiation processes must have a reasonable acceptance angle, must be able to deliver a transmitted contrast ratio above approximately 20:1, and a transmission efficiency above about 30% of the desired polarization, and survive for a useful period of time (at least 1-2 months) in a high intensity environment. It is also desired that the polarizer have a convenient form factor such as a plate format which allows for the most efficient optical geometries to be used. While such a level of performance in the visible spectrum could easily be met by wire-grid polarizer technology or several other polarization technologies, it has proven surprisingly hard to meet even this low performance requirement in the UV.
One solution to this need has been to use a “pile-of-plates” polarizer which is formed by assembling a series of glass plates and positioning the pile at Brewster's angle to the UV irradiation to create a polarized beam through transmission of the P-polarization and reflection of the S-polarization. This approach can deliver the desired optical efficiency and contrast ratio, but it is prohibitively expensive and bulky, and has not proved to be a practical solution.
It had been thought that aluminum wire-grid polarizers similar to those commercially-available for use in the visible and IR would serve to meet this need. Experience, however, has shown that the current state of the art in wire-grid technology is insufficient. Wire-grid polarizers with a grid period down to approximately 100 nm from several manufacturers have been tested in UV applications between 240 nm and 300 nm wavelength and have not been able to meet all the above requirements. In particular, they have not been able to deliver the desired contrast levels for a useful period of time. The fundamental problems appear to be the short wavelength in comparison to the grid period (a ratio of only 2.5:1 at 250 nm) which negatively impacts the contrast and transmission performance, and the harshness of the industrial UV environment which quickly (such as in a matter of a few hours) transforms the aluminum metal wires in the grid into aluminum oxide wires, at which point the polarizer loses its polarization function almost entirely.
Another proposal has been to simply add a separate absorptive layer near a wire-grid polarizer or coating a wire-grid polarizer with an absorptive layer. See U.S. Pat. No. 7,206,059. But such a polarizer uses wires.
Other UV polarizers, such as the Glan Thompson Alpha BBO, while satisfactory in scientific applications, cannot meet the requirements on optical efficiency, acceptance angle, and are also prohibitively expensive for industrial applications. Thus, there does not exist today a fully acceptable and practical UV polarizer that meets the needs of industrial applications of UV light.